Three people died at the scene, after the bombing of the Boston Marathon. But every person transferred to a hospital survived, Dr. Alasdair Conn, chief of emergency services at Massachusetts General Hospital writes in a perspective piece published Tuesday in the Annals of Internal Medicine.

Conn credits the bravery of first responders—medical professionals and bystanders, alike—who risked their personal safety to help those who were injured, taking critical steps to staunch the loss of blood. He notes that the proximity of the blasts to major trauma centers helped. And he mentions another factor: luck.

Conn writes that the timing of the attack was “opportune”:

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[T]he incident occurred at the change of shift. The morning shift was completing the 7 a.m. to 3 p.m. shift; the 3 p.m. to 11 p.m. shift was already in house. On every unit in the hospital the medical, nursing, and support staff stayed to assist however they could – it was as though there was immediate double coverage. It was a Monday; the hospital was relatively open and had not yet filled with the elective cases that tend to occur early in the week. Being a state holiday the scheduled operating list was relatively light but because it was a normal working day the operating rooms were fully staffed; the ORs were also completing their operative schedules for the day. All of these factors contributed, but above all, it was the training and the repeated disaster drills that made the difference.